1. The Field of the Invention
The present invention is directed to an improved NO.sub.x reduction process for controlling of NO.sub.x emissions. More particularly, the present invention is directed to methods for providing a wider temperature window for effective NO.sub.x reduction than existing processes for controlling NO.sub.x emissions.
2. Technology Review
The problems of waste management in the United States urgently require the development of an environmentally acceptable incineration technology, but for one important class of pollutants, nitrogen oxides (commonly referred to as "NO.sub.x "), the presently available NO.sub.x control technology provides only a very limited degree of control.
A survey by the United States Environmental Protection Agency indicates that the United States generates about 140 million metric tons of industrial waste and 230 million metric tons of municipal waste annually. In the past, most waste was disposed of by landfill but such approaches are inherently unsatisfactory because the toxic materials in the waste are not destroyed or rendered innocuous but merely isolated. Recognition of the dangers inherent in disposal by isolation has lead to increasingly tighter control and monitoring of these disposal practices, making them impractical and prohibitively expensive. It is not uncommon for landfills to be closed for reasons of environmental safety even when there are not alternative disposal methods available.
Incineration is potentially the ideal solution to the problem of waste management since toxic organic materials can be completely destroyed and most of the toxic inorganic materials of concern can be converted to an inert glass by operation at temperatures above the ash fusion point. Most of the problems which have given incinerators a poor reputation in the past have satisfactory answers. For instance, emissions of acid gases such as SO.sub.2 and HCl can be controlled by wet scrubbers.
However, control of NO.sub.x emissions from incinerators is a problem to which no presently available technology provides a fully satisfactory answer. While the amount of NO.sub.x produced by burning waste can be minimized by managing the combustion process, waste typically contains substantial amounts of chemically bound nitrogen such that NO.sub.x levels are usually unacceptably high, even with careful control of the combustion process. As a result, some form of post combustion NO.sub.x control technology must be used in incineration processes.
Two types of post combustion NO.sub.x control technologies are presently available, selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR). Applications of SCR to incinerators are generally regarded as nonfeasible because waste contains virtually all possible trace impurities and these impurities can act as catalyst poisons.
Because no better technology currently exits, SNCR processes have been accepted as the best available NO.sub.x control technology for incinerators. In the usual SNCR process, a nitrogen-containing reducing agent, normally either ammonia (NH.sub.3) or urea (H.sub.2 NCONH.sub.2), is contacted with flue gas within a relatively narrow temperature range. The optimum contacting temperature is typically a factor of the reducing agent. A homogeneous gas phase reaction occurs which reduces the NO in the flue gas to molecular nitrogen (N.sub.2) and water (H.sub.2 O). The performance of SNCR in actual incinerator applications has, however, been highly disappointing.
In most applications, the performance of the NO.sub.x reduction processes depends primarily on the available reaction time, i.e., the length of time the flue gas spends in the temperature range suitable for NO.sub.x reduction by the chosen reducing agent. For applications in which the available reaction time is less than 0.2 seconds, NO.sub.x reductions in the 60% to 80% range are typically achieved. For applications in which the available reaction time is greater than 0.2 seconds NO.sub.x reductions in the 80% to 90% range have commonly been achieved.
The design of a modern incinerator provides the post-flame gases with a residence time generally greater than 1.0 seconds in the temperature range appropriate to NO.sub.x reduction processes. Hence, one might expect incinerators to be a very favorable application for selective noncatalytic NO.sub.x reduction. Instead, however, NO.sub.x reduction in incinerators is actually 40% or less.
The poor performance of NO.sub.x reduction processes on incinerators is, in part, a result of the fact that the temperature of the flue gas in incinerators is more highly variable than it is in other combustion systems. Waste is inherently a fuel with a highly variable BTU content. This variability causes the temperature of the flue gases downstream of the combustion zone to be nonhomogeneous in space and to fluctuate in time.
If the temperature of the flue gas is a little too low at the point where the reducing agent is injected, slight or no NO.sub.x reduction occurs. If the temperature is too high, the nitrogen-containing reducing agent (NH.sub.3 or H.sub.2 NCONH.sub.2) has some tendency to oxidize to produce NO, and the net reduction of NO is poor or more NO may even be produced. Because this "temperature window" for the NO.sub.x reduction process is narrow, successful application of the process is always critically dependent on locating the reducing agent injection system at the location at which the average temperature is optimum for the process.
In any application, however, the temperature will be nonhomogeneous, and process performance will be determined by an average over a temperature range. Since this always includes some temperatures which are too high and some which are too low for good NO.sub.x reduction, the practical extent of NO.sub.x control which the process can provide is always significantly less than is achieved in laboratory experiments.
Since the width of the NO.sub.x reduction temperature window increases with increasing reaction time, the longer reaction time available in incinerators compensates, in part, for this difficulty. However, there is an additional problem: the optimum temperature for NO.sub.x reduction may be shifted. For example, as shown in FIG. 1, (quoted from R.K. Lyon and J.E. Hardy, "Discovery and Development of the Thermal DeNO.sub.x Process," Ind. Eng. Chem. Fundam. Vol. 25, page 19, 1986; see also Environmental Science and Technology, Vol. 21, page 232, 1987) hydrogen (H.sub.2 ) mixed with the ammonia shifts the NO.sub.x reduction temperature window to lower temperatures. The magnitude of the temperature shift increases as the amount of H.sub.2 is increased.
This shifting of the temperature window shown in FIG. 1 is a general effect which occurs with other combustible materials, including CO, natural gas, etc. Even though the temperature window may be shifted, the size of the temperature window is not enlarged to a significant degree by the presence of other reducing agents in the combustion effluent stream.
From the foregoing, it is apparent that what is currently needed in the art are methods for controlling NO.sub.x emissions from stationary combustion systems having variable flue gas temperatures. It would also be an advancement in the art to provide methods for controlling NO.sub.x emissions from stationary combustion systems which enlarge the useful temperature window for NO.sub.x reduction.
Such methods for controlling NO.sub.x emissions from stationary combustion systems are disclosed and claimed herein.